1. Field
The present application relates to an imaging device that captures a subject image.
2. Description of the Related Art
In late years, video cameras and electronic cameras using a CCD type or a CMOS type imaging device become widely used. For instance, the CMOS type imaging device includes a plurality of pixels arranged in a two-dimensional matrix form of N rows by M columns and each having a photoelectric conversion part which accumulates a charge in accordance with a light amount of incident light. Further, each pixel includes transistors to output the charge accumulated in the photoelectric conversion part as an electric signal. Further, the imaging device is made up of vertical signal lines and a vertical scanning circuit for reading the electric signals output from the respective pixels for each of the rows, a horizontal output circuit for outputting the electric signals to the outside of the imaging device in a row direction in the order of columns, and the like.
The charge accumulated in the photoelectric conversion part (photodiode) is transferred to an FD (floating diffusion) part. The charge transferred to the FD part is amplified by an amplification transistor and then selected by a selection transistor specifying an address of each pixel, and is read to a vertical signal line on which a source follower circuit is formed by a constant current generator. The electric signal read to the vertical signal line is input to a column amplifier via a coupling capacitor. At this time, a dark signal when light is shielded in which the charge of the FD part is reset is held in a dark signal accumulation part, and an image signal when subject light is incident is held in an image signal accumulation part. Further, a difference between the dark signal held in the dark signal accumulation part and the image signal held in the image signal accumulation part is obtained and output from the imaging device. The circuit is called as a correlated double sampling (CDS) circuit which is capable of obtaining a preferred output by removing noise ascribable to dispersion of the circuits (refer to Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-077642, for instance).
However, in the conventional CMOS type imaging device, grounds of constant current generators disposed on output terminals of the respective vertical signal lines, grounds of constant current generators inside column amplifiers, and grounds of pixels on the respective columns are coupled to a ground line disposed in the row direction at positions on the respective columns. Due to a structure of a mask pattern at the time of manufacturing a semiconductor, the ground line disposed in the row direction is provided with external ground points on left and right ends thereof (left and right ends of the chip), and is coupled to an external ground electrical potential at the external ground points. However, when an imaging device of a relatively large size is used, since a row length is large, a total length of the ground line becomes several tens of milli-orders. Further, operational currents of output circuits of several thousands of vertical signal lines disposed on the respective columns in a parallel manner flow into the ground line, which causes a problem such that an electrical potential difference is generated on the ground line among the column positions.
Further, since grounds of the FD parts of the respective pixels (grounds of pixels) are coupled to ground lines for the pixels disposed in a vertical direction starting from the respective column positions on the aforementioned ground line disposed in the row direction, so that ground electrical potentials of the FD parts of the respective pixels also have an electrical potential distribution in a horizontal direction in which the electrical potentials vary among the columns. Note that the FD part of each of the pixels is reset based on a power supply voltage VDD, and the dark signal and the image signal are also set to be generated based on the reset electrical potential, so that even if a difference is generated among the ground electrical potentials of the FD parts of the respective pixels, no affection is imposed on output signals if there is no temporal change, namely, if a temporally constant state is provided.
However, when a high luminance subject such as illumination is image-formed on a two-dimensionally arranged effective pixel area, outputs of pixel amplifiers (amplification transistors) on a row on which the high luminance subject is image-formed become excessive, and the electrical potential of the vertical signal line becomes lower than an overdrive voltage necessary for operating the constant current generator. As a result of this, the constant current generators on the respective columns cannot maintain a constant current, which reduces a value of current flowing into the ground line.
Further, since the column amplifier is also saturated, an operating point electrical potential of a constant current generator (common current generator) inside the column amplifier is also lowered. As a result of this, a current of the common current generator in the column amplifier is also slightly lowered.
For instance, when the high luminance subject has a size of several hundreds to several thousands of columns, even if the decrease in current flowing though each of the vertical signal lines is several μA, the total decrease in current becomes several mA to several tens of mA. Accordingly, if the row on which the high luminance subject is image-formed is selected, an electrical potential of ground line when the image signal is read becomes slightly lower, by an order of several tens to several hundreds of μV, than an electrical potential of ground line when the dark signal is read. As a result of this, there is generated a slight level difference between the electrical potential of ground line for the dark signal of the row in which the high luminance subject exists and the electrical potential of ground line for the dark signal of the adjacent row in which the high luminance subject does not exist. The level difference becomes a difference in the ground electrical potentials of the FD parts of the respective pixels, so that a slight difference is generated between the outputs of the dark signals. As a result of this, a white smear is generated on both sides of the high luminance subject of the image to be shot.
For instance, in an image of street light at night in which an illumination part is extremely bright and a background is extremely dark, even if an electrical potential difference of the FD parts is very small, it is visually recognized, in the shot image, as a visually recognizable level of smear in a horizontal direction.
A proposition of the present application is to provide an imaging device capable of obtaining an image with high picture quality with no smear even when a high luminance subject exists, by suppressing a variation in electrical potentials of ground line caused by a variation in currents of constant current generators and common current generators in column amplifiers disposed on respective columns to stabilize the ground line disposed in a row direction and ground electrical potentials of pixels coupled to the ground line for each of the columns.